OK this is what I've just found and tested.
The Blu-ray has 2 english subtitle tracks, the first is the full english subs and the forced Navi subs merged together, the second is just the Navi subs. I was using the second sub track because I just wanted the Navi subs the problem being that the subs never clear.
I just tried using the first sub track and checked "Forced Only" and it worked perfectly, I got the Navi subs only and they clear perfectly.
The Blu-ray has 2 english subtitle tracks, the first is the full english subs and the forced Navi subs merged together, the second is just the Navi subs. I was using the second sub track because I just wanted the Navi subs the problem being that the subs never clear.
I just tried using the first sub track and checked "Forced Only" and it worked perfectly, I got the Navi subs only and they clear perfectly.